The need and demand for an accurate, non-invasive method for determining biological attributes of tissue is well documented. Accurate, non-invasive determination of blood glucose, as an example, could reduce many of the complications associated with diabetes. Similarly, accurate, noninvasive determination of various disease states could allow faster, more convenient screening and diagnosis, allowing more effective treatment.
Proposed non-invasive methods for determining biological attributes generally utilize quantitative infrared spectroscopy. Infrared spectroscopy measures the response of a substance to electromagnetic radiation (0.7-25 .mu.m) at various wavelengths. The response can be considered as derived from two categories, diffuse reflectance and specular reflectance. The specular reflectance of a sample is the light which does not propagate into the sample, but rather reflects from the front surface of the sample. This component contains information about the sample at the surface. If the material is homogeneous, this surface reflection can be related to the bulk. While the specular component does not physically appear much like an absorbance spectrum, it can be related to the absorbance spectrum of the bulk material through a transformation called the Kramers-Kronig transformation. The diffuse component is generally considered more useful for sample qualification and quantification than is the specular component. Various approaches have been proposed to emphasize the diffuse component relative to the specular component, but all suffer from shortcomings that limit their utility.
Accordingly, there is a need to improvements in spectroscopic technology that allow greater use of the diffuse component of a substance's response relative to the specular component.